Without a doubt, the most prolific planet hunter to date has got to be NASA’s Kepler mission. Launched into solar orbit on March 7, 2009, Kepler spent four years continuously observing the brightness of almost 200,000 stars in a single patch of sky straddling the border of the constellations Cygnus and Lyra looking for small dips in star brightness indicative of planetary transits (see “First Look at Kepler’s Complete Primary Mission Data Set”). To date, there have been over one thousand confirmed extrasolar planet detections by Kepler with many times more unconfirmed finds currently subject of follow up observations. Unfortunately, after the second of Kepler’s four reaction wheels stop functioning in May of 2013, Kepler was no longer able to point accurately at its target star field bringing its primary mission to an end.
To get around the loss of the two reaction wheels, a new extended mission with a different mode of operations was devised. Kepler now observes a sequence of star fields along the ecliptic for stretches of about 80 days at a time before moving on to the next star field. With this new observation strategy, Kepler can track each star field using its remaining pair of reaction wheels and the slight pressure of sunlight reflecting off of the spacecraft with sufficient accuracy to continue planet hunting. Called “K2”, Kepler’s extended mission with its new observation strategy officially started in May 2014. By the beginning of 2015, the Kepler team started announcing discoveries from the K2 mission including EPIC 201367065d which some claimed was potentially habitable (see “Habitable Planet Reality Check: Kepler’s New K2 Finds”).
Diagram showing the approved and proposed fields for Kepler new K2 extended mission. Click on image to enlarge. (NASA)
As more K2 planet candidates are identified and follow up observations made, the discovery of additional extrasolar planets has been announced. Among these finds is another extrasolar planet known by the cryptic catalog designation EPIC 201912552b. Once again, some have claimed that this distant extrasolar planet is potentially habitable and could be among the most Earth-like planets currently known.
Background
The star EPIC 201912552 (also known as K2-18) is a type M2.8 red dwarf located an estimated 110 light years away in the constellation Leo. EPIC 201912552 is located in the first star field observed during K2 designated Campaign 1 which ran from May 30 to August 21, 2014. Based on an analysis of spectroscopic observations, EPIC 201912552 has a surface temperature of 3503 K, a mass of 0.41 times that of the Sun and a radius of 0.39 times the Sun’s. With a luminosity of just 2% that of the Sun, it is a fairly typical star in the solar neighborhood.
EPIC 201912552 along with 30 other stars in Kepler’s K2 Campaign 1 field came to the attention of the Kepler team because they displayed periodic dips in their brightness indicative of planetary transits. The initial analysis was published by Foreman-Mackey et al. and prompted follow up observations by members of this team and others which were recently submitted for publication by Montet et al.. According to Montet et al., a search through archival images as well as newer data exclude the possibility that the transit signature observed in EPIC 201912552 is a false positive caused by a more distant eclipsing binary background star. A preliminary set of radial velocity measurements obtained in February 2015 also exclude the possibility of the transit event being caused by a close orbiting stellar companion. Indeed, all of the available data indicate that it is highly improbable that EPIC 201912552 has a stellar companion that would complicate the interpretation of the Kepler data. All the data to date suggest that there is less than a 10-4 probability of this planetary detection being a false positive.
Based on the analysis of the available data by Montet et al., the newly discovered EPIC 201912552b has a radius of 2.24±0.25 times that of the Earth (or RE). It is in an orbit with a period of 32.9 days and a semimajor axis of 0.15 AU. Montet et al. calculate that the insolation or effective stellar flux, Seff, of EPIC 201912552b is 1.28±0.28 times that of the Earth. Based on their own calculations, the Planetary Habitability Laboratory (PHL) at the University of Puerto Rico at Arecibo determined that EPIC 201912552b has an ESI (Earth Similarity Index) value of 0.73 making it the 16th most Earth-like planet in their Habitable Exoplanets Catalog as of April 2, 2015.
Artist depiction of the planets in the Planetary Habitability Laboratory’s “Habitable Exoplanets Catalog” as of April 2, 2015. EPIC 201912552b is the planet marked “NEW” in the second row. Click on image to enlarge. (PHL/UPR Aricebo)
Potential Habitability
With the PHL placing EPIC 201912552b on their Habitable Exoplanets Catalog with an above-average ESI rating, I wanted to make an independent evaluation of this world’s potential habitability. Looking at the latest models for the conservative limits of the habitable zone (HZ) from Kopparapu et al., the HZ of EPIC 201367065 for a 5 ME planet would have effective stellar flux values, Seff, ranging from about 1.00 for the inner edge (corresponding to the runaway greenhouse limit) out to about 0.25 (corresponding to the maximum greenhouse limit). With the Seff of 1.28±0.28 calculated by Montet et al., the orbit of EPIC 201912552b is beyond the inner limit of the conservatively defined HZ. Factoring in the current uncertainty in Seff, I estimate that there is only about a 15% chance that EPIC 201912552b is inside this HZ definition.
But in addition to the more conservative definition of the HZ, there are other definitions of the HZ that might still be worth considering. Yang et al. have studied the effects of slow or even synchronous rotation of planets on HZ limits. While the rotation state of EPIC 201912552b is unknown, it could be a slow or even a synchronous rotator as a result of tidal interactions with its sun. Using the models of Yang et al., the inner limit of the HZ of EPIC 201912552b for a slowly rotating planet would correspond to an Seff as high as 1.74. I estimate that there is about a 95% chance that EPIC 201912552b would orbit inside this more optimistic definition of the HZ assuming it is a slow rotator.
Diagram showing the orbit of EPIC 201912552b compared to the HZ limits of the system shown in green. Click on image to enlarge. (PHL/UPR Aricebo)
Setting aside for the moment the question of EPIC 201912552b orbiting inside the HZ, there is the issue of whether or not this world is even a terrestrial planet. A recently published analysis of the mass-radius relationship for extrasolar planets performed by Leslie Rogers strongly suggests that planets transition from being predominantly rocky planets like the Earth to predominantly volatile-rich worlds like Neptune at radii no greater than 1.6 RE (see “Habitable Planet Reality Check: Terrestrial Planet Size Limit”). Using a model based on recent work by Torres et al., I estimate that there is only about a 7% chance that EPIC 201912552b with a radius of 2.24 RE is a rocky planet (see “Habitable Planet Reality Check: 8 New Habitable Zone Planets”). It is much more likely that EPIC 201912552b is a volatile-rich mini-Neptune with little prospect of being habitable in the conventional sense.
This assessment is shared by Montet et al. who never claimed that EPIC 201912552b was a rocky planet or potentially habitable. In fact, because of the relative brightness of EPIC 201912552, Montet et al. specifically state that EPIC 201912552b provides an excellent opportunity to study the atmosphere of a mini-Neptune that has a lower effective stellar flux than other known sub-Neptune size planets. The low probability of EPIC 201912552b being a terrestrial planet combined with questions about whether it orbits inside the HZ make it a very poor candidate for being a potentially habitable planet despite the optimistic claims by the PHL.
Conclusion
Contrary to the claims being made by the PHL, it does not appear that EPIC 201912552b is a good candidate for being a potentially habitable planet given what we currently know about this find. Based on the information in Montet et al., the effective stellar flux is about 28% higher than the conservative inner limit of the HZ as defined by the onset of a runaway moist greenhouse effect as described by Kopparapu et al.. It is only by adopting much more optimistic definitions for the inner limit of the HZ that EPIC 201912552b seems to have a reasonable chance of orbiting in the HZ. Unfortunately, with a radius of 2.24 RE, it is very unlikely that EPIC 201912552b is a rocky terrestrial planet. It is much more probable that it is a volatile-rich mini-Neptune with little prospect of being potentially habitable. Taken together, there is little evidence to support the optimistic assessment by the PHL about the potential habitability of EPIC 201912552b.
While this initial assessment is not very promising, it could change. Additional follow-up observations from the ground as well as by missions like NASA’s TESS (Transiting Exoplanet Survey Satellite) should help to reduce the uncertainties in the properties of this planet possibly tilting the balance towards being potentially habitable planet candidate. Since EPIC 201912552b should produce radial velocity variations on the order of 2 to 3 meters per second, it should be possible to measure its mass (or at very least, place tight upper limits) using ground-based instruments. Once available, this information will be useful in constraining the bulk composition of EPIC 201912552b and would verify the current expectation that it is a mini-Neptune.
Whether it is a large, rocky planet or a mini-Neptune, a warm potentially habitable planet or hot Venus-like world, EPIC 201912552b is an excellent candidate for future study by JWST and other instruments allowing astronomers to probe the properties of extrasolar planets in an unexplored part of planet parameter space. Regardless of the actual status of EPIC 201912552b, such studies will provide real data to help establish the limits of planetary habitability.
Related Reading
“Habitable Planet Reality Check: Terrestrial Planet Size Limit”, Drew Ex Machina, July 24, 2014 [Post]
“Habitable Planet Reality Check: Kepler’s New K2 Finds”, Drew Ex Machina, January 20, 2015 [Post]
“Habitable Planet Reality Check: 8 New Habitable Zone Planets”, Drew Ex Machina, January 8, 2015 [Post]
“First Look at Kepler’s Complete Primary Mission Data Set”, Drew Ex Machina, January 26, 2015 [Post]
“A Review of the Best Habitable Planet Candidates”, Centauri Dreams, January 30, 2015 [Post]
General References
Daniel Foreman-Mackey et al., “A systematic search for transiting planets in the K2 data”, arXiv 1502.04715 (to be submitted to The Astrophysical Journal), February 16, 2015 [Preprint]
R. K. Kopparapu et al., “Habitable zones around main-sequence stars: new estimates”, The Astrophysical Journal, Vol. 765, No. 2, Article ID. 131, March 10, 2013
Ravi Kumar Kopparapu et al., “Habitable zones around main-sequence stars: dependence on planetary mass”, The Astrophysical Journal Letters, Vol. 787, No. 2, Article ID. L29, June 1, 2014
Benjamin T. Montet et al., “Stellar and Planetary Properties of K2 Campaign 1 Candidates and Validation of 18 Systems, Including a Planet Receiving Earth-like Insolation”, arXiv 1503.07866 (submitted to The Astrophysical Journal), March 26, 2015 [Preprint]
Leslie A. Rogers, “Most 1.6 Earth-Radius Planets are not Rocky”, The Astrophysical Journal, Vol. 801, No. 1, Article id. 41, March 2015 [Preprint]
Guillermo Torres et al., “Validation of Twelve Small Kepler Transiting Planets in the Habitable Zone”, The Astrophysical Journal, Vol. 800, No. 2, Article id. 99, February 2015 [Preprint]
Jun Yang et al., “Strong Dependence of the Inner Edge of the Habitable Zone on Planetary Rotation Rate”, The Astrophysical Journal Letters, Vol. 787, No. 1, Article id. L2, May 2014
Habitable Exoplanets Catalog, Planetary Habitability Laboratory (University of Puerto Rico at Arecibo) [Link]
Meanwhile, another one on the PHL list appears to have failed the “does the planet exist?” check: looks like Kapteyn b isn’t there.
Robertson, Roy & Mahadevan, “Stellar activity mimics a habitable-zone planet around Kapteyn’s star” arXiv:1505.02778
Please, don’t forget these interesting references:
Jérémy Leconte et al. “Asynchronous rotation of Earth-mass planets in the habitable zone of lower-mass stars” arXiv http://arxiv.org/abs/1502.01952
Ramses Ramirez et al. “The Habitable Zones of Pre-Main-Sequence Stars” arXiv http://arxiv.org/abs/1412.1764
“Contrary to the claims being made by the PHL, it does not appear that EPIC 201912552b is a good candidate for being a potentially habitable planet given what we currently know about this find. ”
If the claim is their estimate of their first attempt to optimistically filter out habitability candidates, it is a good candidate for inclusion (7 % likelihood of being habitable). I don’t think they do an other claim than looking at habitability cnbdidates, e.g. their name shouldn’t be taken as a guarantee that they present pessimistically filtered candidates or only such.
Is their filter method good? Time will tell. but my feeling is that someone needs to use a pessimistic filter to optimize first searches for results. The PHL approach is more of theoretical interest. (Oddly enough, since Mendez started out with tying his filter methods to net primary productivity of Earth. That should have increase resolution of the set of most interesting candidates, analogous to the recent methods that picks out super-habitable (potentially more habitable than Earth) candidates. Oh, well.)
Well, it seems that water vapor has been discovered in its atmosphere:
https://www.nationalgeographic.com/science/2019/09/first-water-found-in-habitable-exoplanets-atmosphere-hubble-kepler-k2-18b/